Traditionally, heavy vehicles have been left idling when at rest to maintain power to ancillary equipment. Environmental regulations, including new anti-idling laws, now restrict this practice and many vehicles now carry a second smaller engine or auxiliary power unit (APU) to run the ancillary equipment when the vehicle's prime mover is shut down during rest stops or layovers.
Thus, systems are known in which an auxiliary engine mounted to the vehicle's frame serves the purpose of maintaining vital functions in the vehicle when the main engine is shut down. These vital functions include electrical generators, heating (preheating or maintaining heat) of the main engine electrically and/or by transfer of coolant from the APU to the main engine and other vehicle components, cooling of personal areas of the vehicle by means of electrically powered hermetically sealed air conditioners or air conditioners powered by means of transfer of a refrigerant from a compressor on the APU to an evaporator in the cab (split system air conditioner), and powering, directly or indirectly, hydraulic systems for linear or rotating motors.
These systems have several disadvantages. As the auxiliary engine must be mounted on the outside of the vehicle, the compressor, in case of a split air conditioning system, and the generator, both being attached to the engine, will be exposed to adverse weather conditions and salt spray and dirt from the road. This causes corrosion and in the case of the generator electric insulation problems which can cause electric shock. The transmission lines for refrigeration, in the case of a split system, must be opened for installation, vacuum pumped and charged with the refrigerant. These lines are prone to wear and leaks. This makes installation and repair expensive and it requires more refrigerant than a hermetically sealed system and increases the risk of refrigerant leaks to the environment. In the case of a generator, the power is transmitted through wires which, when subjected to wear and humidity, can cause shorts and shocks. In case of hydraulic systems, an oil tank is obviously required and these are costly, heavy and space consuming, and in some cases a hydraulic oil cooler with a separate radiator and fan system is also required. Hydraulic oil leaks contaminate the environment and create a fire hazard. Hydraulic oil is expensive.